A typical conventional transverse flux inductor comprises a pair of induction coils. A material to be inductively heated is placed between the pair of coils. For example, in FIG. 1, the coil pair comprises coil 101 and coil 103, respectively located above and below the material, which may be, for example, metal strip 90, which moves continuously through the pair of coils in the direction illustrated by the arrow. For orientation, a three dimension orthogonal space is defined by the X, Y and Z axes shown in FIG. 1. Accordingly the strip moves in the Z direction. The gap, gc, or opening, between the coil pair is exaggerated in the figure for clarity, but is fixed in length across the cross section of the strip. Terminals 101a and 101b of coil 101, and terminals 103a and 103b of coil 103, are connected to one or more suitable ac power sources (not shown in the figures) with instantaneous current pluralities as indicated in the figure. Current flow through the coils creates a common magnetic flux, as illustrated by typical flux line 105 (illustrated by dashed line), that passes perpendicularly through the strip to induce eddy currents in the plane of the strip. Magnetic flux concentrators 117 (partially shown around coil 101 in the figure), for example, laminations or other high permeability, low reluctance materials, may be used to direct the magnetic field towards the strip. Selection of the ac current frequency (f, in Hertz) for efficient induced heating is given by the equation:
  f  =      2    ×          10      6        ⁢                  ρ        ⁢                                  ⁢                  g          c                                      τ          2                ⁢                  d          s                    
where ρ is the electrical resistivity measured in Ω·m; gc is the gap (opening) between the coils measured in meters; τ is the pole pitch (step) of the coils measured in meters; and ds is the thickness of the strip measured in meters.
The classical problem to be solved when heating strips by electric induction with a transverse flux inductor is to achieve a uniform cross sectional (along the X-axis), induced heating temperature across the strip. FIG. 2 illustrates a typical cross sectional strip heating profile obtained with the arrangement in FIG. 1 when the pole pitch of the coils is relatively small and, from the above equation, the frequency is correspondingly low. The X-axis in FIG. 2 represents the normalized cross sectional coordinate of the strip with the center of the strip being coordinate 0.0, and the opposing edges of the strip being coordinates +1.0 and −1.0. The Y-axis represents the normalized temperature achieved from induction heating of the strip with normalized temperature 1.0 representing the generally uniform heated temperature across middle region 111 of the strip. Nearer to the edges of the strip, in regions 113 (referred to as the shoulder regions), the cross sectional induced temperatures of the strip decrease from the normalized temperature value of 1.0, and then increase in edge regions 115 of the strip to above the normalized temperature value of 1.0.
There is a need for a transverse flux induction heating apparatus, either in the configuration of the induction coils, or compensators used with the induction coils, that will reduce induced edge overheating and increase induced heating in shoulder regions of the work piece.